TM 5-852-4/AFM 88-19, Chap. 4
stresses over a relatively large area should be used.
(1) Temperatures at floor level in an
These are essentially the same principles as used in
unheated building will depend on such factors as roof
design of pile foundations in frozen ground (para 4-8).
and wall insulation, degree of ventilation, roof and wall
h. Grouted anchors may be set in ice-free rock
exterior reflectivity, and seasonal percent sunshine and
are best determined experimentally in comparable
in conventional drill holes. The drill holes will require
buildings in the same area. Temperatures at floor level
preheating before grouting if the rock is frozen. Grouted
in an unheated building fully open to the outside air may
anchors may be installed in ice-free rock without
usually be assumed to average the same as standard
preheating if the rock is warmer than 30F, if high-early
shaded meteorological station air temperatures. For this
or other fast setting cements are used, provided the
situation, slab-on-grade construction without insulation
temperature of the ground is greater than 60F at the
can be employed if a mat of non-frost-susceptible
time of placement and the annular thickness of the grout
material is used, as illustrated in figure 4-95, sufficiently
around the anchor rod is at least 2 1/2 inches (i.e.,
thick to contain seasonal freeze and thaw.
With
diameter of hole 6 inches or greater for 1-inch rod).
modification of the thickness of the non-frost susceptible
Enlarged bells may be augered and integrally poured
mat as required by local climate, and possibly radiant
with the normal grouted rod anchors to provide additional
heat input through windows, this design can be used
anchor capacity.
Because of the low ground
under any building in any seasonal frost area or under
temperatures, lead was used to grout anchorages into
any fully ventilated, unheated building in any frost area.
bedrock during construction of a major antenna at Thule,
Closed unheated buildings with no more than nominal
Greenland, in the 1950's to avoid the uncertainties of
ventilation tend to have warmer average annual
using portland cement mortar under these conditions.
temperatures, particularly from absorption of solar heat
However, such practice is not recommended today in
in summer. This is qualitatively illustrated in figure 4-40.
light of present techniques and capabilities for analyzing
Degradation of permafrost under this closed, insulated
such problems.
i.
building (with ineffective foundation ventilation system)
Mass-gravity anchors have the advantage
only slowly decreased after discontinuance of heating. If
in cold regions that they are positive, can always be
the average annual temperature in the building is warm
counted on, and are free of the risk of creep. However, if
enough to cause degradation of permafrost, the design
placed on top of frost-susceptible soils, the risk of frost
in figure 4-95 will no longer be suitable if the foundation
heave and consequent variable anchor tension must be
soils will settle significantly on thaw. Temperatures in an
considered.
Mass-gravity anchors are particularly
unheated earth-covered igloo or below-ground structure
suitable where clean, granular or even bouldery soils
may usually be assumed to average the same as the
exist which can be easily excavated and handled for
ground temperatures at the average depth of the facility.
complete or partial burial of cast-in-place or precase
The possibility that lighting or other electrical facilities
anchor units. Deadmen can also be advantageously
and body heat may introduce significant amounts of heat
used to bear against frozen soil, but excavation costs are
into closed facilities should be considered.
usually quite high.
j.
(2) Where non-frost-susceptible material
In permafrost areas it is usually preferable
is scarce or expensive or where very deep frost
to install anchors in the winter in order to cause as little
penetration would require an uneconomical thickness of
permanent thermal disturbance of the permafrost as
mat, the following alternatives to the designs in figure 4-
possible while at the same time assuring rapid
95 may be considered.
development of the design anchor capacity.
(a) Use of under-slab insulation to
k. For permanent anchors in frozen ground,
reduce the thickness of non-frost-susceptible fill
design should be predicated on whichever is controlling:
required.
ultimate strength or holding creep within acceptable
(b) Use of a structural floor supported
limits. Factors of safety should not be less than those
by footings or piles sufficiently above the ground so that
specified in paragraphs 4-4 or 4-8h depending on the
it will be isolated from frost heave. This system can also
type of stressing. Failure of an anchor by pull-out is
be used to provide foundation ventilation to insure the
more likely to be catastrophic than the failure of a footing
coldest possible temperature conditions at the ground
in settlement would be. Therefore, the factor of safety
surface.
against actual pull-out should also be at least equivalent
(c) Use of a gravel floor of nominal
to the factor of safety in the supported structure based
thickness directly on the natural soil, accepting resultant
on ultimate strength.
frost heave.
4-15. Foundations for non-heated facilities.
Foundations of non-heated facilities may involve special
design problems, considerations or requirements.
a. Non-heated buildings.
4-158
Previous Page
